Geoffrey W. Heard

Detecting extinction risk from climate change by IUCN red list criteria

Anthropogenic climate change is a key threat to global biodiversity. To inform strategic actions aimed at conserving biodiversity as climate changes, conservation planners need early warning of the risks faced by different species. The IUCN Red List criteria for threatened species are widely acknowledged as useful risk assessment tools for informing conservation under constraints imposed by limited data. However, doubts have been expressed about the ability of the criteria to detect risks imposed by potentially slow-acting threats such as climate change, particularly because criteria addressing rates of population decline are assessed over time scales as short as 10 years. We used spatially explicit stochastic population models and dynamic species distribution models projected to future climates to determine how long before extinction a species would become eligible for listing as threatened based on the IUCN Red List criteria. We focused on a short-lived frog species (Assa darlingtoni) chosen specifically to represent potential weaknesses in the criteria to allow detailed consideration of the analytical issues and to develop an approach for wider application. The criteria were more sensitive to climate change than previously anticipated; lead times between initial listing in a threatened category and predicted extinction varied from 40 to 80 years, depending on data availability. We attributed this sensitivity primarily to the ensemble properties of the criteria that assess contrasting symptoms of extinction risk. Nevertheless, we recommend the robustness of the criteria warrants further investigation across species with contrasting life histories and patterns of decline. The adequacy of these lead times for early warning depends on practicalities of environmental policy and management, bureaucratic or political inertia, and the anticipated species response times to management actions.

Assessing detection probabilities for the endangered growling grass frog (Litoria raniformis) in southern Victoria

Assessment of the efficacy of survey techniques for determining species occurrence is crucial for the validation of wildlife survey data. We analysed repeated site-survey data for adults and larvae of the growling grass frog (Litoria raniformis) in order to estimate probabilities of detection for the species using alternative survey techniques. The estimated probability of detecting adults of L. raniformis at occupied sites using diurnal searches was much less than 1.0 (0.107; 95% credible interval: 0.045, 0.192). The estimated probability of detecting adults using nocturnal spotlight searches was considerably higher, but still less than 1.0 (0.696; 95% credible interval: 0.585, 0.796). These results indicate that nocturnal searches are a much more efficient and reliable means of detecting the presence of adult L. raniformis than diurnal searches, but detection using either technique is less than certain. The probability of detecting tadpoles of L. raniformis using either funnel-trapping or dip-netting techniques was estimated at 0.350 (95% credible interval: 0.151, 0.567). Together, these results indicate that reliance on single-site visits during surveys for this species is likely to result in severe under-estimation of the proportion of sites that are actually occupied. We urge other workers to use repeated site-survey data and appropriate methods of data analysis to assess and report probabilities of detection when documenting the results of wildlife surveys.

Wetland characteristics influence disease risk for a threatened amphibian

Identifying determinants of the probability and intensity of infections is important for understanding the epidemiology of wildlife diseases, and for managing their impact on threatened species. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis, has decimated populations of some amphibians. However, recent studies have identified important environmental constraints on the disease, related to the pathogens physiological tolerances. In this study, we identified several intrinsic and extrinsic determinants of the probability and intensity of chytrid infections for the threatened growling grass frog (Litoria raniformis) in southeastern Australia, and used mark-recapture to estimate the effect of chytrid infections on the probability of survival of these frogs. Water temperature and salinity had negative effects on both the probability and intensity of chytrid infections. We coupled models of the infection process with a model of the effect of chytrid infections on the probability of survival to assess variation in the impact of chytridiomycosis between wetlands with differing temperature and salinity profiles. Our results suggest that warm, saline wetlands may be refuges from chytridiomycosis for L. raniformis, and should be priorities for protection. Our results also suggest that management actions that increase water temperature (e.g., reducing canopy shading) and salinity (e.g., complementing inflows with groundwater) could be trialed to reduce the impacts of chytridiomycosis on this species. This and other recent studies highlight the value of research on environmental risk factors for chytridiomycosis.

Refugia and connectivity sustain amphibian metapopulations afflicted by disease

Metapopulation persistence in fragmented landscapes depends on habitat patches that can support resilient local populations and sufficient connectivity between patches. Yet epidemiological theory for metapopulations has largely overlooked the capacity of particular patches to act as refuges from disease, and has suggested that connectivity can undermine persistence. Here, we show that relatively warm and saline wetlands are environmental refuges from chytridiomycosis for an endangered Australian frog, and act jointly with connectivity to sustain frog metapopulations. We coupled models of microclimate and infection probability to map chytrid prevalence, and demonstrate a strong negative relationship between chytrid prevalence and the persistence of frog populations. Simulations confirm that frog metapopulations are likely to go extinct when they lack environmental refuges from disease and lose connectivity between patches. This study demonstrates that environmental heterogeneity can mediate host-pathogen interactions in fragmented landscapes, and provides evidence that connectivity principally supports host metapopulations afflicted by facultative pathogens.

Structure and fragmentation of growling grass frog metapopulations

Metapopulations occur in fragmented landscapes, and consist of demographically-independent populations connected by dispersal. Nevertheless, anthropogenic habitat fragmentation may be fatal to metapopulations, as it disrupts dispersal and gene flow, and undermines the balance between population extinction and colonization. Understanding the extent to which particular land-use practices disrupt dispersal and gene flow is therefore crucial for conserving metapopulations. We examined the structure and fragmentation of metapopulations of the endangered growling grass frog (Litoria raniformis) in an urbanizing landscape in southern Australia. Population clustering analyses revealed three distinct genetic units, corresponding to the three wetland clusters sampled. Isolation-by-distance was apparent between populations, and genetic distance was significantly correlated with the presence of urban barriers between populations. Our study provides evidence that urbanization fragments metapopulations of L. raniformis. Managers of L. raniformis in urbanizing landscapes should seek to mitigate effects of urbanization on dispersal and gene flow.

Microhabitat preferences of the endangered Growling Grass Frog * Litoria raniformis in southern Victoria

We examined nocturnal microhabitat preferences of the endangered Growling Grass Frog Litoria raniformis in lotic and lentic environments in southern Victoria, Australia. Data were obtained during surveys of six wetland sites during the summer of 2003. At all sites the observed distribution of frogs amongst microhabitat categories differed from their availability, as assessed by sampling of random points. Frogs were located most often on bare soil, bare rock or leaf litter when on land, and on floating, submergent and emergent vegetation in aquatic situations. Non-metric multidimensional scaling and analysis of similarities (ANOSIM) were used to compare the structural attributes of microhabitats used by L. raniformis to those of random points. In both the riparian and aquatic zones of the study sites, microhabitats used by these frogs differed from random points in their degree of vertical structural complexity. Whilst our data may be biased by the observability of frogs in different microhabitats, this st...

Visible implant alphanumeric tags as an alternative to toe-clipping for marking amphibians - a case study.

The removal of toes in unique combinations (‘toe-clipping’) has historically been the most popular technique for individually marking amphibians. However, recent studies have questioned the utility and ethics of this technique, and provided impetus for research on the efficacy of alternative methods. In this study, the efficacy of Visible Implant Alphanumeric (VIA) tags for marking juvenile growling grass frogs (Litoria raniformis) was assessed with reference to the fundamental assumptions of the mark–recapture approach. Three questions were investigated: (1) What is the probability of tag loss? (2) What is the probability of tag misidentification, and does it vary between frogs or observers, or with observer experience? and (3) To what extent does tagging influence survival or growth? Laboratory and field trials demonstrated that the probability of tag loss for juvenile L. raniformis was ~0.07 (95% CI: 0.03–0.13) when tags where implanted on the dorsolateral region of the thorax, eight times lower than that estimated for tags implanted on the thigh (mean 0.59; 95% CI: 0.29–0.86). Tags were rarely misidentified by naive observers (mean probability of tag misidentification <0.01), but tag read accuracy varied amongst observers. In the laboratory, tagging did not adversely influence survival or growth of L. raniformis during the 12-week study period. Given that modification of the techniques used here may eliminate tag loss, and that variation in tag read accuracy between observers may be easily avoided, it is concluded that VIA tags represent a promising alternative to toe-clipping for marking L. raniformis and other amphibians.

Integrating variability in detection probabilities when designing wildlife surveys: a case study of amphibians from south-eastern Australia

Occupancy-based monitoring programs rely on survey data to infer presence or absence of the target species. However, species may occupy a site and go undetected, leading to erroneous inference of absence (‘false absence’). If detectability is influenced by the time of year or weather conditions, survey protocols can be adjusted to minimize the chance of false absences. In this study, detection probabilities for three amphibian species from south-eastern Australia were modelled using a Bayesian approach. For aural surveys, we compared basic models, which only included effects of survey date, duration and time of day on detection, to models including additional effects of weather. Model selection using deviance information criterion (DIC) suggested that the basic model was the most parsimonious for Crinia signifera, while models including relative humidity and water temperature were most supported for Limnodynastes dumerilii and L. tasmaniensis respectively. When predictive performance was assessed by cross validation, DIC results were largely matched for C. signifera and L. dumerilii, while models of detection for L. tasmaniensis were indistinguishable, AUC scores suggesting inadequate performance. We show how results such as these can be used to design surveys, developing protocols for individual surveys and estimating the number of surveys required under those protocols to achieve a threshold cumulative probability of detection. Conservation managers can use these models to maximize the efficiency of surveys. This will improve the accuracy of occupancy data, and reduce the risk of misdirected conservation actions resulting from false absences.

Manipulating wetland hydroperiod to improve occupancy rates by an endangered amphibian: modelling management scenarios

Summary 1.Environmental managers have the difficult task of ensuring species persistence despite considerable uncertainty about their response to management. Spatially-explicit population models provide one solution for simulating the dynamics of species and evaluating alternative management regimes. 2.We used a Bayesian model to investigate wetland occupancy dynamics of the endangered growling grass frog Litoria raniformis at a wastewater treatment plant in southern Victoria, Australia. We coupled prior information from earlier research on this species with our survey data to quantify the effects of patch-scale variables and connectivity on the probabilities of occupancy, population extinction and colonisation. Hydroperiods of 13 sites were experimentally enhanced to bolster occupancy rates by L. raniformis. We used simulations to assess the extent to which the enhanced hydroperiod regime improved the viability of the focal metapopulation. 3.Occupancy rate increased by 15% among the enhanced sites in 2013–2014, whereas the rate of occupancy among unenhanced sites fell by 11% in that year. Forward simulation using the dynamic occupancy model suggested that the minimum occupancy rate across the metapopulation would be 18% higher if the enhanced hydroperiod regime was retained over the next 20 years. 4.Mean posterior effects of patch-scale variables and connectivity on the occupancy dynamics of L. raniformis were consistent with the prior effect in all cases, with only small changes to the size of these effects. There was no clear effect of water chemistry on occupancy dynamics. 5.Synthesis and applications. This work suggests that managing the hydroperiod of constructed wetlands can be an effective tool for the conservation of amphibians, and demonstrates the utility of spatially-explicit models for assessing metapopulation viability. We encourage managers to experimentally test the efficacy of manipulating patch-scale variables to improve occupancy rates within amphibian metapopulations. This article is protected by copyright. All rights reserved.

Genetic structure and diversity of the endangered growling grass frog in a rapidly urbanizing region

Two pervasive and fundamental impacts of urbanization are the loss and fragmentation of natural habitats. From a genetic perspective, these impacts manifest as reduced genetic diversity and ultimately reduced genetic viability. The growling grass frog (Litoria raniformis) is listed as vulnerable to extinction in Australia, and endangered in the state of Victoria. Remaining populations of this species in and around the city of Melbourne are threatened by habitat loss, degradation and fragmentation due to urban expansion. We used mitochondrial DNA (mtDNA) and microsatellites to study the genetic structure and diversity of L. raniformis across Melbournes urban fringe, and also screened four nuclear gene regions (POMC, RAG-1, Rhod and CRYBA1). The mtDNA and nuclear DNA sequences revealed low levels of genetic diversity throughout remnant populations of L. raniformis. However, one of the four regions studied, Cardinia, exhibited relatively high genetic diversity and several unique haplotypes, suggesting this region should be recognized as a separate Management Unit. We discuss the implications of these results for the conservation of L. raniformis in urbanizing landscapes, particularly the potential risks and benefits of translocation, which remains a contentious management approach for this species.